THS3001 Datasheet, PDF (19/32 Pages) Texas Instruments – 420-MHz HIGH-SPEED CURRENT-FEEDBACK AMPLIFIERS

THS3001, THS3002
420-MHz HIGH-SPEED CURRENT-FEEDBACK AMPLIFIERS
APPLICATION INFORMATION
SLOS217A – JULY 1998 – REVISED JUNE 1999
recommended feedback and gain resistor values
The THS300x is fabricated using Texas Instruments 30-V complementary bipolar process, HVBiCOM. This
process provides the excellent isolation and extremely high slew rates that result in superior distortion
characteristics.
As with all current-feedback amplifiers, the bandwidth of the THS300x is an inversely proportional function of
the value of the feedback resistor (see Figures 26 to 34). The recommended resistors for the optimum frequency
response are shown in Table 1. These should be used as a starting point and once optimum values are found,
1% tolerance resistors should be used to maintain frequency response characteristics. For most applications,
a feedback resistor value of 1 kΩ is recommended – a good compromise between bandwidth and phase margin
that yields a very stable amplifier.
Consistent with current-feedback amplifiers, increasing the gain is best accomplished by changing the gain
resistor, not the feedback resistor. This is because the bandwidth of the amplifier is dominated by the feedback
resistor value and internal dominant-pole capacitor. The ability to control the amplifier gain independent of the
bandwidth constitutes a major advantage of current-feedback amplifiers over conventional voltage-feedback
amplifiers. Therefore, once a frequency response is found suitable to a particular application, adjust the value
of the gain resistor to increase or decrease the overall amplifier gain.
Finally, it is important to realize the effects of the feedback resistance on distortion. Increasing the resistance
decreases the loop gain and increases the distortion. It is also important to know that decreasing load
impedance increases total harmonic distortion (THD). Typically, the third-order harmonic distortion increases
more than the second-order harmonic distortion.
Table 1. Recommended Resistor Values for Optimum Frequency Response
GAIN
1
2, –1
–2
5
RF for VCC = ± 15 V
1 kΩ
680 Ω
620 Ω
560 Ω
RF for VCC = ± 5 V
1 kΩ
750 Ω
620 Ω
620 Ω
offset voltage
The output offset voltage, (V_OO) is the sum of the input offset voltage (V_IO) and both input bias currents (I_IB) times
the corresponding gains. The following schematic and formula can be used to calculate the output offset
voltage:
RF
IIB–
RG
+
–
VIO
+
VO
RS
ǒ ǒ ǓǓ ǒ ǒ ǓǓ ^IIB+
+ ) " ₎ ) VOO VIO 1
RF
RG
IIB RS 1
RF
RG
" IIB– RF
Figure 48. Output Offset Voltage Model
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